Why anacron

Cron can be used to
run a task on regular basis. It cannot anyway run passed task.
Example i want a a task to be run at 10:20, but at this time the
computer was off and now it's 11:00, this means the task will wait
tell next time the condition is met again.

Anacron, on the
other hand, will run an action after a specified amount of time
beginning when the computer is on.

#Here I added my job
to be launched 5 minutes after power on of the computer every day

#you can give it any
name (here "ka"). The script or other command must be in
PATH above

1 5 ka
ka_update &>> /root/log"

Anacron log file

A file under the
/var/spool/anacron directory, with the same name as the job. This
file will contain a single line that indicates the last time the job
was executed.

[root@localhost ~]#
ls -ld /var/spool/anacron/*

-rw-------. 1 root
root 9 May 30 19:02 /var/spool/anacron/ka

[root@localhost ~]#
cat /var/spool/anacron/ka

20170530

[root@localhost ~]#
date

Wed May 31 07:30:05
CEST 2017

Some very useful options

Taken simply from man anacron

Testing anacrontab

-T
Anacrontab testing. Tests the /etc/anacrontab configuration file for
validity. If there is an error in the file, it is shown on the
standard output and Anacron returns the value of 1. Valid
anacrontabs return the value of 0.